This invention relates to wind turbines, and in particular, to apparatus and methods for limiting the speed of a wind turbine rotor.
Wind turbines conventionally comprise a tall tower of circular or polygonal cross-section with a wind turbine, usually in an elongated nacelle, mounted at the top. The wind turbine has a large multi bladed propeller attached at one end and adapted to rotate about the horizontal axis. The propeller is connected through a system of gears within the nacelle to an electrical generator also contained within the nacelle. The nacelle is arranged to rotate about a vertical axis (yaw) into the wind. All of the parts driven by the turbine mechanically are contained within the body of the wind turbine or nacelle. Electrical power generated within the nacelle is carried down the tower and away to its destination by cables and/or rotary electrical connectors.
Many small and large wind turbines depend on the load of the generator to keep the rotor speed within the design limits of the rotor and rotor blades. Some wind turbines have safety brakes designed to stop the rotor. Prudent designers of wind turbines take into consideration the failure of one or more drive train components. Should the gearbox fail, for example, a brake on the high speed shaft of the generator would be unable to stop or slow the rotor. Should a brake fail, there may be no way to stop the turbine in a high wind.
One of the safest and surest ways to keep wind turbine rotor RPM within safe limits is to utilize some means of aerodynamic rotor speed control. One of the most common ways of accomplishing this is to turn the blades so that the blade airfoil is incapable of producing lift. Because this requires the entire blade to be turned, the method requires substantial actuating force and large and expensive bearings at the blade roots. The mechanism for turning the blades must also be failsafe and not dependent on outside sources of power, which may fail precisely at the time when needed most. This redundancy increases complexity and cost, often making it uneconomical for small turbines.
Another common method of rotor overspeed control is to only turn a small section of the blade, usually the blade tip. This method has the advantage of requiring much lower forces. Additionally, the blade root sections can be rigidly affixed to the rotor hub without the need for expensive pitch bearings. Because the outer portions of the blade have the greatest speed, turning the blade tips 90° to the apparent wind direction creates substantial drag and a large moment which is very effective in slowing the rotor. For a fixed pitch rotor, blade tip brakes are an efficient and economical means of rotor overspeed control. In practice, only the outer 10% to 15% of the blade needs to be turned.
For a constant speed wind turbine which uses a grid tied induction or synchronous generator, or a turbine wherein the rotor RPM is controlled by the load of the generator, there is no need for aerodynamic speed control during normal operation. By proper and careful design of the drive train and control system, the load of the generator can be made to keep the rotor speed within design limits. With such a design the only time aerodynamic speed control needs to be implemented is upon failure of the generator load or a failure of the drive train. In the case of a grid connected wind turbine, this can happen as a result of utility power failure. It can also happen to any wind turbine if some drive train component fails or if the brake fails. Because these events should be relatively rare, the aerodynamic speed control which is the subject of this invention need only be put into service in the event of a failure of the drive train, control system or brake.